Computer player

@dejay has done some excellent work in getting gtktactoe into a fully-functional state. The code is *not* particularly pretty, but it works well and doesn't have any obvious bugs or major crashes.

The major changes in this pull request are as follows:

* Allow the game to be restarted independently from the application

* Added a menu to the interface to facilitate new features

* Added a working AI opponent, who is strong enough to always draw or win